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•Creep rupture strength of austenitic stainless steels have been predicated.•Newly developed physically based models for creep cavitation have been used.•Results for ductile and ...brittle creep rupture are compared.
Creep rupture can happen in two ways, brittle and ductile creep rupture. Brittle creep rupture of austenitic stainless steels proceeds with the nucleation, growth and coalescence of grain boundary cavities. A creep cavity nucleation model has been developed previously, which considers cavity nucleation at particles and sub-boundary corners due to grain boundary sliding. A modified constrained cavity growth model has been used to describe the cavity growth behavior with combination of the cavity nucleation models. In this paper, the brittle creep rupture has been analyzed by combining the cavity nucleation and growth models. The physically based models where no adjustable parameters were involved were used to predict the brittle creep rupture strength. On the other hand, previously developed basic models for ductile creep rupture based on exhaustion of the creep ductility have been used. The creep rupture strength of common austenitic stainless steels has been predicted quantitatively by taking both ductile and brittle rupture into account. The predicted rupture times for ductile rupture are longer than those for brittle rupture at high stresses and low temperatures with a reversed situation at low stresses and high temperatures. This reproduces the characteristic change in slope in the creep rupture curves.
The creep properties of Mg alloys are critical for their high temperature applications. But for Mg–Gd alloys, there is a lack of systematic research on the creep behavior and microstructure evolution ...above 250 °C. In this study, the high temperature tensile creep behavior and microstructure evolution of the T6 state Mg-15Gd (wt.%) metal mold casting alloy were investigated at temperatures from 235 to 300 °C and stresses from 50 to 90 MPa. The experimental alloy was composed of α-Mg matrix, elliptic shape β′ precipitates, cuboid-shaped GdH2 phase and a small amount of M5Gd phase. It was found that the creep strain and creep rate of the experimental alloy increased but the creep life decreased with increasing the creep temperature and the applied stress. At a fixed creep temperature of 260 °C, the calculated stress exponent n value was about 2.8 in the applied stress range of 50–90 MPa, suggesting that the creep deformation may be controlled by dislocation slip. Meanwhile, under a defined creep stress of 50 MPa, the calculated activation energy Q value of the alloy was about 108 kJ mol−1 in the temperature range of 235–300 °C, indicating that the pipe diffusion may be a dominant affecting factor. During the creep process, the α(Mg) grain size coarsened and the quantity, size and type of the precipitated phases changed with three-stage: β′ (cbco) →β1 (fcc) →β (fcc). Observation of creep microstructure evolution under 50 MPa and 260 °C confirmed that the creep mechanism was dislocation glide and the creep fracture mechanism was brittle transgranular cleavage fracture.
To better understand the creep mechanical behavior of salt rock under very long-term tests and more accurately predict the long-term deformation of the salt caverns, a series of long-term uniaxial ...creep tests lasting for 21000 h (875 days) were carried out. The results show that the creep strain and creep rate of salt rock increase with the increase of axial stress. The initial creep stage under lower stresses lasted for 4.3–8.3 months, which was much higher than that under high stresses. Predicting the true creep rate of the salt cavern based on the test results of the creep rate in this paper is more accurate than the existing research results. A new nonlinear creep-damage constitutive model is proposed by introducing the nonlinear damage body and using the fractional derivative theory. Using the presented model and the classic creep constitutive models to fit the experimental data, it is found that the presented model best characterizes the creep evolution characteristics of salt rock. The fitting function obtained from the experimental data at different times predicts the final creep results, revealing that the longer the test duration, the more accurate the predicted results. The research in this paper provides the necessary basis for intensive investigation and prediction of the long-term creep mechanical behavior of salt storage caverns.
Herein the tensile creep behavior and microstructure evolution of an as‐cast Mg–9.82Gd–0.38Zr alloy under different stresses at 250 °C are investigated. The results of creep test show that the creep ...strain and steady creep rate increase with the creep stress at 250 °C. The fitted stress exponent n value (4.4) and transmission electron microscopy analysis suggest that the steady creep is dominated by the dislocation climb and glide mechanisms together. At the primary stage, due to the segregation of the Gd content, a fine‐strengthening β′ phase precipitates near the α‐Mg grain boundary. Then the transformation of β′ precipitates to a needle‐like β phase, coarsening, and growth of the β phase toward the intragranular direction gradually deteriorate the creep resistance of the experimental alloy. Moreover, two kinds of precipitate‐free zones (PFZs) appear at the end of the secondary creep stage and the PFZs nearly perpendicular to the applied stress direction widen gradually because of the directional diffusion. The nucleation and propagation of creep crack along the cracked eutectic phase and PFZs lead to the greatly rising creep rate at the end of the tertiary creep stage, which eventually results in the intergranular creep fracture of the experimental alloy.
During the creep process of an as‐cast Mg–9.82Gd–0.38Zr alloy at 250 °C under 80 MPa, the β′ precipitates are transformed into needle‐like β precipitates and precipitate‐free zones (PFZs) widen gradually due to the directional diffusion. The nucleation and propagation of creep crack along the cracked eutectic phase and PFZs lead to creep fracture of the experimental alloy.
•A model is proposed for the relationship between indentation and uniaxial creep.•Effect of primary creep on the conversion factor is addressed.•Expression of the conversion factor is deduced under ...spherical and impression indentation.•Predicted indentation creep data can match well with uniaxial creep results.
In this work, a theoretical model is developed to address the effect of primary creep on the relationship between indentation and uniaxial creep tests. By considering the expansion of a spherical cavity in the materials following the primary creeping law, a general expression of the conversion factor is deduced during the primary creep stage. For creep tests with spherical and flat punch indenters, a closed-form formula is specified for the conversion factor that is determined by the stress exponent and hardening exponent. By further considering the difference of the critical time at the onset of steady-state creep deformation between uniaxial and indentation creep tests, it becomes available to convert the indentation creep data into uniaxial creep results during the steady-state creep stage. The rationality and accuracy of the proposed model are verified by comparing with the experimental data of Sn37Pb solder, AZ31 Mg alloy and Succinonitrile. Furthermore, the comparison with previous theoretical models indicates that the ignoring of primary creep effect could lead to the deviation of converted indentation creep data from uniaxial creep results.
Dolomitic limestone is the main surrounding rock material in Yangzong tunnel engineering; the instantaneous mechanical properties and creep behaviors of limestone are significant for stability ...evaluation during the stages of tunnel excavation and long-term maintenance. Herein, four conventional triaxial compression tests were carried out to explore its instantaneous mechanical behavior and failure characteristics; subsequently, the creep behaviors of limestone subjected to multi-stage incremental axial loading at the confinements of 9 MPa and 15 MPa were studied by employing an advanced rock mechanics testing system (i.e., MTS815.04). The results reveal the following. (1) comparing the curves of axial strain-, radial strain-, and volumetric strain-stress under different confining pressures shows that these curves present a similar trend, whereas the stress drops during the post-peak stage decelerate with the increase in confining pressure, suggesting that the rock transits from brittleness to ductility. The confining pressure also has a certain role in controlling the cracking deformation during the pre-peak stage. Besides, the proportions of compaction- and dilatancy-dominated phases in the volumetric strain-stress curves differ obviously. Moreover, the failure mode of the dolomitic limestone is a shear-dominated fracture but is also affected by the confining pressure. (2) When the loading stress reaches a creep threshold stress, the primary and steady-state creep stages occur successively, and a higher deviatoric stress corresponds to a greater creep strain. When the deviatoric stress surpasses an accelerated creep threshold stress, a tertiary creep appears and then is followed by creep failure. Furthermore, the two threshold stresses at 15 MPa confinement are greater than that at 9 MPa confinement, suggesting that the confining pressure has an obvious impact on the threshold values and a higher confining pressure corresponds to a greater threshold value. Additionally, the specimen's creep failure mode is one of "abrupt" shear-dominated fracturing and is similar to that under a conventional triaxial compression test at high confining pressure. (3) A multi-element nonlinear creep damage model is developed by bonding a proposed visco-plastic model in series with the Hookean substance and Schiffman body, and can accurately describe the full-stage creep behaviors.
A creep test performed on a Landes salt sample during one year and a half is described. During the first year, a 0.6 MPa axial load is applied to the sample. At the end of this one-year phase, strain ...rate (9×10−12s-1) is much faster than the strain rate extrapolated from high-stress tests. Steady state strain rate is not reached. In an attempt to reach steady state strain rate “from below”, a 0.9 MPa load is applied during two days before restoring the initial load (0.6 MPa). After the load is restored, reverse creep is observed first (strain rate sign changes before vanishing to zero after a few hours). Then, strain rate increases to reach 5×10−12s-1 after five months, slower than the strain rate before the load change. Commonly accepted constitutive laws can explain this effect, which provides a lower and an upper bound for steady state strain rate. This note presents a method to determine such bounds.
The tensile creep behavior of as-extruded and T6 treated Mg–8Gd–3Y–0.3Zr (GW83) alloy was estimated under temperature at 170–200 °C and stress at 40–70 MPa. The results showed that the primary creep ...strain and steady state creep rate increased, while the duration of steady state creep decreased with increasing temperature and stress. The primary creep constitutive model of GW83 alloy kept an Andrade power law ε = βtk with k value of 0.24–0.6 for as-extruded alloy and 0.13–0.49 for T6 alloy, respectively. The steady state creep plastic deformation of as-extruded and T6 state alloys can be expressed by Arrhenius constitutive equations with stress exponents 5.6–5.9 and 8.9–9.8, creep activity energy 124.9–135.6 kJ/mol and 152.2–195.8 kJ/mol, respectively. The tensile creep resistance of GW83 alloy improved significantly by T6 heat treatment, the effect of temperature on steady state creep rate was relatively moderate, and the tensile creep plastic deformation of GW83 alloy mainly was controlled by applied stress. The steady state creep deformation mechanism of as-extruded GW83 alloy was regulated by Mg self-diffusion conducted by dislocation climb before heat treatment, and then transformed to dislocation cross-slip after T6 treated. The recrystallization and texture evolution of (0002) and (10-10) played a non-ignorable effect on the tensile creep deformation. The creep rupture life can be predicted by steady state creep rate that will be controlled by temperature under low and intermediate stress and by stress under larger applied stress, respectively. The creep fracture surfaces with coarse dimples and tear ridges presented as ductile transgranular fracture features with the secondary cracks occurred.
The mechanical characteristics of salt rock are investigated using uniaxial compression tests and creep tests in a salt diapir located in the south of Iran. To investigate how the microstructure ...evolution of salt rock due to creep test affects the stress-strain behavior, the crept samples were again tested at a constant displacement rate. The test results showed that the axial peak stress, axial peak strain, and modulus of elasticity gradually increase with increasing strain rate. The stress-strain curves were typical for a ductile material characterized by moderate strain hardening. The creep strain and creep rate increased with increasing axial stress. The Burgers viscoelastic creep model agreed quite well with the experimental creep data. The microstructure observations indicated that grain-boundary cracking and grain boundary sliding were the dominant deformation mechanisms. Under the uniaxial compression test, the crept samples showed a slightly higher modulus of elasticity compared to the intact salt samples. Finally, it seems that free dislocations increased during the creep test led to strain hardening of salt samples through the uniaxial compression test.
This study seeks to control the microstructure of Ti-6Al-4V alloy produced by a selective laser melting process using only heat treatment, while also improving the high temperature creep behavior. ...Initial microstructure observation showed a fully martensite structure for the as-fabricated specimen and a fine widmanstätten structure for the heat-treated specimen. Hardness tests indicated that the hardness of the material decreased somewhat, but the anisotropy of the mechanical properties (common in additive manufactured materials) disappeared. Compression tests at 500°C indicated that the yield strength of the as-fabricated specimen was approximately 930MPa, while in the heat-treated specimen it was approximately 557MPa. High temperature compressive creep tests showed that the heat-treated specimen had relatively low creep strain and low steady-state creep rate (superior creep resistance) in all stress ranges, which is the opposite of the compressive yield strength results. After creep deformation, the microstructural evolution of the as-fabricated and heat-treated specimens was compared, and the microstructure changed much more extensively in the as-fabricated specimen. Correlations between microstructures, hardness, strength and creep properties were discussed based on these results.
